1. Field of the Invention
The present invention relates to a connecting structure and a frequency adjusting method therein, and, more particularly, to improvements in a connecting structure for circuits operating at a microwave or millimeter wave frequency band.
2. Description of Relevant Art
An electrical device is produced by combining various circuits and devices, for example. Practically, circuits are installed in the electrical device by means of forming a printed circuit board for each circuit, placing the printed circuit boards on a mother board through spacers or the like, and connecting each printed circuit board to the circuit on the mother board.
Conventionally, each printed circuit board is connected to the mother board (printed circuit board) or devices with a connecting member of metal, such as a wire or a ribbon, so that conduction is established between the terminals of circuits or the like to be connected by linking both ends of the connecting member to the respective terminals electrically and mechanically by means of soldering or ultrasonics.
However, the conventional connecting structure using the above connecting member has the following problems. That is, when the connection is established with the above connecting member in the range of a microwave or a millimeter wave having a short wavelength, the circuit constant varies in response to the length and shape of the connecting member, and also in response to a quantity of soldering or the connecting position when the connecting member is bonded to the terminals, which causes the transmission state to become unstable.
Also, when the desired circuit characteristics cannot be obtained due to the foregoing causes, characteristic adjusting processes of various kinds have to be carried out by, for example, partially removing the connecting member or soldering or providing an auxiliary member. However, most of the characteristic adjusting processes rely on one""s experience and involve a complicated procedure. Therefore, these processes are not suitable for mass-producing products of uniform quality.
In addition, because the terminals and connecting member are closely adhered to each other, if the circuit arrangement or devices need to be changed after manufacturing, the connecting member is cut to separate the connected printed circuit boards, and reconnected to a replaced printed circuit board or the like. This cutting procedure makes the replacement job difficult.
Further, if cuttings from the cut connecting member are not removed completely, the residual cuttings may adversely affect the circuit characteristics. Moreover, heating for adhering the connecting member, or removing the cuttings, may undesirably cause a change in the shape and dimension of the circuit pattern or separation of the same.
The present invention has been made to solve the above problems, and therefore, has as an object to provide a new connecting structure and a new frequency adjusting method in the connecting structure.
Another object of the present invention is to provide a non-contact connecting structure for high frequency circuits operating at microwave or millimeter wave frequencies, with which a connection between circuit elements can be readily established without requiring high accuracy in assembly and dimension. Also, components can be readily replaced after the connection is established while suppressing a change in the connection state that possibly occurs during replacement. The connecting structure is also stable under temperature-change and capable of protecting devices on the circuit. A further object is to provide a frequency adjusting method in such a connecting structure.
In order to achieve the above and other objects, in a first aspect of the present invention, there is provided a connecting structure for first and second high frequency circuit elements (corresponding to first and second circuit boards 2 and 3 in the embodiment discussed below) each provided with a dielectric substrate having a transmission line (corresponding to strip lines 4a and 4b in the embodiment) made out of a conductive film on a top surface thereof. The first and second high frequency circuit elements are placed in such a manner as to secure a predetermined gap therebetween. A connecting terminal pattern (corresponding to patch portions 5a and 5b in the embodiment described below) constituting a resonator is formed continuously with the transmission line on the top surface of the high frequency circuit elements, and a supporting member is placed on a top surface of the connecting terminal pattern formed on the first high frequency circuit element. A parasitic element made of a dielectric material is cantilevered by the supporting member, and a free end side of the parasitic element is placed above the connecting terminal pattern formed on the second high frequency circuit element while securing a predetermined space between the parasitic element and the connecting terminal pattern on the second high frequency circuit element.
The high frequency circuit element referred to herein means literally an element used in a high frequency circuit, and concrete examples include a circuit board such as a printed circuit board, and other devices. Also, the high frequency referred to in the present invention means the frequency of a microwave or a millimeter wave, for example, and may be an even higher frequency.
The experimental results reveal that when the parasitic element is cantilevered by the supporting element as above such that both ends of the parasitic element are overlaid on the two connecting patterns that will be connected, impedance matching can be achieved in a broad band, which results in a satisfactory connection state within in a high frequency range. This is because a high frequency signal is allowed to propagate due to electromagnetic coupling between the connecting terminal patterns (resonators) and parasitic element.
In the case that the circuits forming the circuit elements are connected, or the circuit and device are connected, processing tolerance and assembly tolerance make it quite difficult to complete assembly while securing an exact gap as designed, which causes a gap between the elements to vary. In the present invention, however, by using the parasitic element, the coupling is enhanced and the passing band is broadened, thereby making it possible to maintain a stable transmission state even when the gap varies to some degree.
Further, because a space is formed between at least the free end of the parasitic element and connecting terminal pattern, a capacitor is formed across this space. Thus, even if static electricity or an abnormal potential, such as a surge, is propagated onto the circuit, it is cut at the connecting structure of the present invention, and will not be propagated extensively to the subsequent stages. Moreover, because the second high frequency circuit element and parasitic element are not fixed to each other, replacement at the second high frequency circuit element side is particularly easy.
Furthermore, because the parasitic element is cantilevered, and therefore, is not directly fixed to the second high frequency circuit element, the transmission line (circuit pattern) made out of the conductive film will not be separated due to stress that occurs as a result of heat contraction in response to a temperature change.
The supporting member may have a high dielectric constant. However, according to a second aspect of the present invention, it is preferable that the supporting member is made of a material having a low dielectric constant. The material having a low dielectric constant referred to herein means a material having a dielectric constant lower than that of the dielectric substrate. Besides those generally referred to as the materials having a low dielectric constant, materials having a dialectic constant greater than 1 and not greater than 3 are preferable.
By using the material having a low dielectric constant, loss in the supporting member can be reduced. Also, one end of the parasitic element is fixed onto the material having a low dielectric constant while the parasitic element and connecting terminal patterns are electromagnetically coupled in a secure manner. This further ensures the effect of suppressing variation in characteristics in response to positional displacement of the connected portion that possibly occurs during replacement of the circuit and device.
In addition, examples of the material having a low dielectric constant include glass and other various kinds of materials. However, according to a third aspect of the present invention, it is preferable that the material having a low dielectric constant is a foam material (urethane foam in the embodiment). Because the foam material has resilience, it can also function as a buffer. In other words, if the parasitic element and the high frequency circuit elements have different coefficients of thermal expansion, differences in thermal expansion and contraction in response to a temperature change are absorbed by the supporting member and will not be delivered to the other. Consequently, no stress is generated at the connected portion in response to a temperature change, and separation of the transmission line can be prevented as much as possible.
Further, according to a fourth aspect of the present invention, it is advantageous that a length of the resonator is xc2xc of an effective wavelength (xcexg). In other words, when the parasitic element is cantilevered, the length of the resonator can be set not to xcex/2 but to xcex/4, thereby making it possible to downsize the entire circuit substrate. It should be appreciated, however, that the length does not have to be exactly xc2xc, and may be slightly larger or smaller than xc2xc as long as desired transmission characteristics can be achieved. In short, the idea discussed herein is approximation to xc2xc, with which substantially the same effect can be achieved.
The parasitic element can be made of resin and other various kinds of materials. However, according to a fifth aspect of the present invention, it is preferable that the parasitic element is made out of a ceramic substrate. In other words, when ceramic is used, the dimension and shape vary so little that the transmission state is stabilized and a change in the transmission state between the products can be suppressed. Further, because most of the dielectric substrates are based on alumina, by employing the ceramic substrate (alumina, for example), there can be expected an effect that substantially the same coefficient of thermal expansion is given to both the parasitic element and circuit boards.
According to a sixth aspect of the present invention, it is preferable that substantially the same coefficient of thermal expansion is given to the parasitic element and dielectric substrate. When the coefficients of thermal expansion are equal, even if a temperature changes during manufacturing or actual use, occurrence of stress at the connected portion through the supporting member can be suppressed as much as possible. Consequently, separation of the transmission line can be prevented effectively. Thus, it should be understood that the term xe2x80x9csubstantially the samexe2x80x9d means a range in which the above discussed effect can be achieved, and the idea discussed herein may allow a slight difference in the coefficients of thermal expansion between the parasitic element and dielectric substrate.
Further, according to a seventh aspect of the present invention, it is advantageous that substantially the same resonance frequency is set in the parasitic element and the connecting terminal pattern. When arranged in this manner, the electromagnetic coupling at the used frequency, and hence the transmission characteristics, can be enhanced.
In addition, according to an eighth aspect of the present invention, it is advantageous that a length of the parasitic element is between or equal to 1.4 and 1.5 times greater than a length of the connecting terminal pattern.
In other words, in the case that the dielectric substrate and parasitic element are made of the same material, such as alumina, or the materials have substantially the same dielectric constant, it is preferable to set the length of the parasitic element between or equal to 1.4 and 1.5 times greater as described above, because by so doing, the electromagnetic coupling is enhanced. The present invention is used in connecting the circuits and devices, and for this reason, deterioration of impedance at the connected portion has to be suppressed. Thus, it is preferable to set the length in the above range, because the return loss of approximately xe2x88x9220 dB can be secured. It should be appreciated that when the specifications covering the required characteristics are less restrictive, the length may be outside the above range (between or equal to 1.4 and 1.5 times greater), and there is no problem in practical use when the set length is outside the above range.
A ninth aspect of the present invention provides a frequency adjusting method of the connecting structure according to any of the first through seventh aspects, wherein a used frequency is adjusted by changing the length of the parasitic element employed.
In other words, the experimental results have revealed that changing the length of the parasitic element causes displacement of the central frequency. The connecting structure of the present invention must meet the demands that it can be used not only in a broad frequency band, but also under the circumstances where the most satisfactory characteristics are achieved. In addition, the frequency may be displaced to the outside of the broad band for some reason. Thus, under these circumstances, the length of the parasitic element is changed to displace the central frequency, so that the desired transmission characteristics can be achieved.
The length can be changed in various manners, for example, by preparing a plurality of the parasitic elements of different lengths and choosing the optimal one, or removing or cutting a part of the parasitic element after it is attached. In the case of preparing a plurality of the parasitic elements of different lengths, the parasitic element is temporarily attached one by one and the characteristics are measured for each until the desired characteristics are achieved. Once the desired characteristics are achieved, the temporarily attached parasitic element is employed and attached properly. In particular, the characteristics vary per lot unit when the high frequency circuit elements or the like are manufactured. Thus, prior to the assembly using a new lot, the characteristics may be checked by the above temporal attachment to determine a desired length, so that the parasitic element having the length thus determined is used for assembly using that lot.